Street design includes provision for the collection and discharge of storm water. Drainage system components such as pipe, catch basins, and inlets are considered integral street improvement elements as are curbs, sidewalks and pavement.

When property development includes the installation of new or replaced paving or other impervious surfaces, there may be a need to improve existing drainage systems in the street to accommodate the additional stormwater flows that will be incurred. This may trigger a requirement to make grading improvements, or to extend or upgrade existing storm drains, inlets, and catch basins, and there may be requirements for detention and/or treatment facilities triggered by the City Stormwater Ordinance. Factors such as the amount of impervious surface involved, the project location, the availability and capacity of existing infrastructure, among others, play a role in determining these requirements.

All required drainage improvements shall be designed and constructed in accordance with the City of Seattle Standard Plans and Specifications which establish acceptable materials, dimensions, locations, installation and testing requirements, and other requirements for pipelines, manholes, connections, detention facilities and other system improvements. A Client Assistance Memo (CAM) is under development to provide design guidance to supplement the Standards.

Seattle Public Utilities (SPU) is responsible for the plan review of new street drainage and stormwater treatment facilities, as well as for the review of project impacts to existing SPU infrastructure.

Standard Plans 200 Series

Storm Drains and Street Drainage

Drainage review: A drainage review will be required for street improvements which involve more than 750 square feet of land disturbing activity.

Drainage : Shall be provided for improved streets and alleys and shall include catch basins and inlets that discharge to a public combined sewer, public storm drain, or other discharge point approved by Seattle Public Utilities (SPU).

Surface drainage : Establishing appropriate street grades is very important for drainage. Standards for cross slopes and longitudinal slopes are important for vehicular and pedestrian safety as well as surface water conveyance. A standard street cross section diagram can be viewed here and shall have a centerline crown elevation. Cross slope minimums shall be met to comply with the American with Disabilities Act (ADA), and also to ensure surface drainage gets to the gutter and flows down to drainage pickups. Streets shall generally have a centerline crown elevation, with some exceptions, such as super-elevated streets The surface drainage must be picked up before every intersection at the uphill side of any ADA wheel chair ramp. Drainage pickups may either be an inlet or catch basin depending on the site conditions. Refer to Section 4.4 Grading and Section 4.5 Design Cross Section for more information.

Public storm drains : A new public storm drain may be required when

• there is no available public storm drain or public combined sewer, and there is no acceptable discharge point;
• to achieve adequate capacity; and
• when new pavement is installed over the permanent storm drain location and it is probable that the street will be a future public storm drain route.

New public storm drains shall be sized to handle all upstream tributary area from the drainage basin in which they are located. They shall be designed to be continuous with existing and future storm drain pipes that are or will be part of the storm drainage system. The engineer shall provide the hydraulic calculations used in the pipe design for review by SPU. Refer to Chapter 6.4 Natural Drainage Systems for guidance on providing a natural drainage conveyance system.

Storm drain pipes: New storm drain pipes shall be designed in accordance with the criteria shown in SPU Client Assistance Memo (CAM) Design Guidelines for Public Storm Drain Facilities (currently under development).

Storm drain water quality systems : Are generally site specific designs. For City owned systems, functional and operational needs for the site may require specific improvements and access requirements.

Stormwater detention : In areas served by a combined sewer, storm drains of inadequate capacity, or those that discharge into a riparian corridor, stormwater detention will be required for new and replaced impervious areas exceeding 2000 square feet, in accordance with the Stormwater, Grading and Drainage Control Code . Storm Drain detention systems shall be either an off-line system or an in-line system as approved by SPU. Standard pipe material for detention systems located within the City right-of-way and owned and operated by SPU shall be concrete or ductile iron pipe.

Stormwater treatment : If the new impervious areas exceed 5000 square feet, or the combined new and replaced impervious areas exceed one acre, there will be requirements for stormwater treatment. ( See City Stormwater Ordinance for street specific impervious surface requirements). Public treatment facilities are installed in the street, and upon successful inspection they are turned over to SPU for operation and maintenance. Guidance for the design of these facilities is provided in published SPU Director’s Rules.

Stormwater code compliance for street improvements : Street improvements shall comply with the requirements of the stormwater code. This includes street paving and sidewalk projects.

Sanitary and Combined Sewers

Location of public sewers : Public sewers shall be located in the centerline of the right of way and street at a minimum 12 foot of depth. The minimum pipe diameter shall be 8 inches. The standard pipe material shall be concrete pipe. All tees shall be 6 inches minimum. Sometimes it is advantageous to lay new side sewers to the curb line or property line whenever possible to avoid digging a newly paved street.

Extensions or upsizing of sanitary or combined sewers : May be required in order to provide service for projects that develop or rehabilitate certain properties. For public sewer extensions, all new sewers shall be designated sanitary. In the State of Washington, it is illegal to build a new combined sewer. For sewer extensions in combined sewer areas, the developer shall be required to build both a sanitary sewer and a storm drain with the storm drain temporary tied into the combined sewer downstream so that the combined sewer area may be separated in the future.

Detention of stormwater flows : Additionally, new or replaced paving or other impervious surfaces trigger code requirements for detention of additional stormwater flows to combined sewers.

Side sewers and service drains : The pipeline between the building and the sewer or storm drain main is a side sewer or service drain, respectively. All side sewer and service drain work within the right-of-way requires a Street Use Utility Permit issued by DPD on behalf of SDOT.

Minimum grade, pipe size and materials, connection details, installation and testing requirements : Side sewers and service drains shall be designed and installed in accordance with the City of Seattle Standard Plans and Specifications, 2008.

Ownership : Side sewers and service drains are owned and maintained by the property owner. The minimum grade is 2%, 6” minimum size; pipe material shall be according to the Seattle Side Sewer Code. New core taps shall be per the core cut procedure.

Use of existing side sewers : It is possible to use existing side sewers in lieu of a new connection in some cases. It is highly recommended that the property owner’s contractor conduct a video inspection and advise the property owner prior to deciding.

Build-overs : For build-overs for SPU owned sewer and drainage appurtenances, the applicant shall follow the guidance of SPU’s build-over CAM.

Inspection : Inspection of drainage improvements may be by DPD, SDOT or both. DPD will not issue the building occupancy permit until the pipe is formally accepted by SPU. The installer should arrange with SPU to inspect the pipe and obtain approval for the installation prior to pavement restoration.